In connection with known incremental position measuring devices, usually at least two phase-shifted periodic analog scanning signals are output in the course of scanning a periodic graduation structure by a suitably designed scanning unit. These signals are further processed in a known manner in an evaluation unit located downstream of the scanning unit for determining the relative position of the periodic graduation structure and the scanning unit. The scanning unit and the periodic graduation structure may be connected with two elements of a machine tool respectively which can be moved with respect to each other, for example. A numerical control may be used as the evaluation device.
The accuracy of position determination by such a position measuring device depends on the quality of the periodic scanning signals generated. Depending on the physical scanning principle employed, a number of errors of different types may exist. For example, in an optical measuring system, inaccuracies in reflective and transmissive graduation structures have a negative effect on signal quality. In other scanning principles, for example in magnetic position measuring devices, the scanning signals output do riot always meet the desired requirements. For example, scanning distance may vary or temperature variations can affect the magnetically sensitive detector elements, etc.
Certain types of error have an interfering effect particularly during subsequent interpolation by an evaluation unit, i.e., the further electronic division of the analog scanning signals. However, evaluation units are typically programmed to presume that the analog scanning signals have an ideal shape, or that an ideal relationship exists between the signals. The various types of errors include different amplitude values of the two phase-shifted scanning signals, a phase shift which differs from the presupposed phase shift, or possibly present d.c. voltage offsets of the two periodic scanning signals. In the case of conventional measuring systems, the mentioned phase shift is 90.degree., however, in connection with interferential three-grating measuring systems, there can be an ideal phase shift of 120.degree. between three different scanning systems.
Besides the option of optimizing actual signal gains, there have been other attempts to automatically and electronically correct such errors in position measuring devices which generate periodic analog scanning signals. For example, an electronic correction for optical position measuring devices is disclosed in Wang, C. et al., "Auto-Correction of Interpolation Errors in Optical Encoders" Proc. of SPIE, vol. 2718, 1996, pp. 439 to 447. The Wang et al. article proposes transmitting the analog scanning signals by a suitable A/D converter to a micro-controller in which correction parameters are determined on the basis of a known algorithm. Such a known correction algorithm is described, for example, in Heydemann, P. L. M., "Determination and Correction of Quadrature Fringe Measurement Errors in Interferometers" in Applied Optics, vol. 20, no. 3,1981, pp. 3382 to 3384; and by Birch, K. P., "Optical Fringe Interpolation with Nanometric Accuracy" Precision Engineering, vol. 12, no. 4, 1990, pp. 195 to 198. With D/A converters downstream of the microcontroller, the correction parameters reach an analog switching circuit through which the analog periodic scanning systems can be affected. Therefore the corrected scanning signals which correspond to the presumed ideal signal shape are present on an output side of the analog switching circuit and can be further processed in known electronic evaluation devices.
Such a correction method has disadvantages because, as a rule, the analog switching circuit through which the analog scanning signals are affected, also contains certain errors. Among these are, for example, undesired offset errors, or an undefined signal amplification. These errors, however, are not accounted for in the definition of the correction parameters, or respectively the corresponding adjustment signals, and therefore continue to falsify the analog scanning signals as before in an undesired way. It is additionally required to tune the sensitivity of the correction parameters, or respectively the corresponding adjustment signals generated by the microcontroller, to the analog switching circuit, which is problematic in case of possibly existing errors in the analog switching circuit.
It has further been shown to be disadvantageous that the selection of the data employed for the determination of correction parameters must be checked by software. Such a check of the data requires a certain amount of time for calculation, which in turn limits the speed of the proposed correction method which is particularly important for high processing speeds.